1. Field of the Invention
The present invention relates to a motor driving apparatus that controls drive of a motor, and particularly to a motor driving apparatus including a sensor for detecting a position of a rotor.
2. Description of the Related Art
A method for providing good drive control of a stepping motor in a wide speed range has been proposed which performs open-loop control in a low driving speed range and performs feedback control in a high driving speed range.
FIG. 13 shows a conventional control method being performed by combining the open-loop control and the feedback control. In FIG. 13, a horizontal axis shows time, and a vertical axis shows rotational speed of the motor or driving speed of a driven member that is driven by the motor. The rotational speed of the motor and the driving speed of the driven member are hereinafter collectively referred to as a “driving speed”). A period tp1 and a period tp2 are acceleration periods, a period tp3 is a constant speed period where a constant speed Vcm is maintained, and a period tp4 and a period tp5 are deceleration periods. Moreover, the period tp1 is the acceleration period in the open-loop control, the period tp2 is the acceleration period in the feedback control, the period tp4 is the deceleration period in the feedback control, and the period tp5 is the deceleration period in the open-loop control. In addition, a point Pc1 is a switching point from the open-loop control to the feedback control, and a point Pc2 is a switching point from the feedback control to the open-loop control.
During acceleration, the open-loop control increases the driving speed to a predetermined speed Vc1, and then is switched to the feedback control. On the other hand, during deceleration, the feedback control decreases the driving speed to a predetermined speed Vc2, and then is switched to the open-loop control. The predetermined speeds Vc1 and Vc2 are set lower than a speed at which the stepping motor loses steps.
Japanese Patent Laid-Open No. 60-28800 discloses a control method that retrieves an acceleration/deceleration pattern to be used, from a memory, according to a driving step number (rotation amount) of a stepping motor, the memory prestoring plural acceleration/deceleration patterns.
However, in the control method shown in FIG. 13 and the control method disclosed in Japanese Patent Laid-Open No. 60-28800, the acceleration pattern used in the acceleration period tp1 where the open-loop control is performed is decided to one pattern regardless of a magnitude of a load acting on the motor in that period tp1. Therefore, a speed increase rate in the acceleration pattern (a slope of the graph shown FIG. 13) is set low so as to be able to drive the motor without losing steps even in a state where a supposed maximum load acts on the motor. In other words, even though a low load allows a high speed increase rate in the acceleration pattern, a low speed increase rate is always set.
Moreover, the point Pc1 shown in FIG. 13 where the open-loop control is switched to the feedback control is decided to one point, a low speed increase rate in the acceleration pattern needs a long time until the driving speed increases up to the driving speed (switching speed) Vc1. Thus, a start timing of high-speed drive of the motor with the feedback control is delayed.